1. Field of the Invention
The field of the invention is water treatment, or, more specifically, my invention relates to the treatment of water used in systems wherein water is circulated for repeated use, such as evaporative cooling systems.
2. Description of Related Art
Evaporative cooling water is used to cool various liquids or gases, in cooling systems using an evaporative cooling unit, a heat exchanger and a source of makeup water piped together in a circulating line. The heat exchanger warms the circulating water, which is circulated back to the cooling tower. The warmed water cascades down inside the cooling tower, and cools by evaporation, due to the fresh air flowing counter-current through the tower fill section.
Such evaporative cooling systems, of which cooling towers are one example, operate on the principle that the latent heat of vaporization of the water being evaporated subtracts energy from the system, thus, reducing the temperature of the remaining water in the system. Only some of the water is evaporated, however, and the salts in the remaining water are manifested in increasing dissolved solids. The most common dissolved salts in domestic water are bicarbonates, chlorides, and sulfates of calcium, magnesium and sodium. When a water containing calcium bicarbonate is heated, as in cooling of air conditioning systems or other equipment, the heat in the heat exchanger, will strip off one molecule of carbon dioxide, rendering the remaining calcium salt to calcium carbonate (limestone), also known as “scale.” This precipitate, the scale, is less soluble in warm water than in cool water and has very poor thermal conductivity, thus reducing heat exchanger efficiency. The scale also becomes less soluble as the pH of the circulating water increases. A higher rate of solids precipitation occurs in a high pH environment.
To maintain a concentration of solids that reduces the formation of scale, fresh water is added from the makeup water source to replace the water lost due to evaporation. Also, water with high concentrations of solids are “wasted” or “blown down” through the system drain to a sewer or ditch, and this must be replaced with makeup water as well.
Total makeup water volume is the sum of evaporated water (E) plus blowdown water (BD). For evaporative cooling systems a “concentration ratio” (CR) is defined as the volume of makeup water divided by the volume of blowdown water. A large concentration ratio is achieved through reduction in the blowdown volume. Restated for blowdown volume, the equation is:BD=E/(CR−1)
Several processes are used to chemically treat evaporative cooling water in order to reduce scale, a number of which are discussed in U.S. Pat. No. 5,730,879. Various combinations of chemicals and inorganic acids are used, but, for example, the current state-of-the-art limits a cooling system using makeup water with 150 parts per million hardness, to a concentration ratio of less than 6, when the total circulating system has a total maximum alkalinity of 600 ppm. In this situation, a cooling tower evaporating 5 million gallons of water per day, with a concentration ratio of 6, wastes 1 million gallons of water per day. In such a system, the blowdown water usually contains between 600 and 900 ppm hardness, and requires blowdown after approximately 6 volumes of system water have been evaporated (referred to as “cycles of concentration”).
U.S. Pat. No. 5,730,879, referenced above, utilizes a sidestream system for treating a portion of the total evaporative cooling water, in an effort to reduce scale formation at the heat exchanger. Cation resin is used to remove water hardness. The resin beads must be regenerated with salt, acid or caustic (depending on the resin used), and then water washed to remove calcium ions. The regeneration solutions become the blowdown.
Other known treatments add chemicals directly into the primary circulation line and have some success in lowering scale formation and increasing the concentration ratio and cycles of concentration. The additive most commonly used is sulfuric acid, which converts calcium carbonate into the more soluble calcium sulfate. Both calcium carbonate and calcium sulfate precipitate more readily as the temperature of the evaporative cooling water increases. In addition, the relatively high concentration of sulfuric acid renders it potentially corrosive. Sulfuric acid can also be hazardous to handle.
While the foregoing processes and treatments may function generally with respect to the purposes for which they were designed, they would not be as suitable for the purposes of the present invention, as hereinafter described. For example, such processes do not provide what is needed, that is an effective process for reducing the amount of blowdown water and scale formation at the heat exchanger, using relatively small amounts of chemicals.